Nitroguanidine based gas generant containing mica

ABSTRACT

Gas generants in an inflator have nitroguanidine as a fuel and mica as a slagging agent. A gas generant with nitroguanidine has many desirable properties such as little hygroscopicity, a high conversion efficiency, and a suitable burn rate. Mica is a beneficial ingredient to the gas generant because it reduces the amount of undesirable gases as well as reduces the amount of solid combustion particles from escaping the inflator

FIELD OF THE INVENTION

[0001] The present invention generally relates to gas generatingcompositions utilized for inflation of an occupant safety restraints inmotor vehicles. In particular, the present invention relates to gasgenerating compositions containing the fuel, nitroguanidine and theparticulate reducer, mica.

BACKGROUND OF THE INVENTION

[0002] Vehicle airbags have been developed to afford protection tooccupants involved in a car crash. During a crash, an airbag is filledwith inflation gas, and the inflated airbag protects an occupant byacting as a cushion. A common method of providing inflation gas to avehicle airbag is by utilizing a pyrotechnic inflator. Simply, apyrotechnic inflator operates by the rapid burning of a gas generant toproduce inflation gas.

[0003] Previously, sodium azide was commonly utilized as a gas generantfor inflators because the burning of sodium azide produces gas rich innitrogen gas. However, gas generants containing sodium azide have anumber of drawbacks such as dangerous decompositional explosion,formation of explosive compounds by the reaction with heavy metals, andenvironmental pollution caused by mass disposal. For all of thesereasons, the person skilled in the art has sought to replace (avoidselecting) sodium azide as the fuel for the gas generant.

[0004] Non azide based gas generants have been developed to overcome thedrawbacks associated with azide based gas generants. Co-owned U.S. Pat.No. 6,071,364 to Canterberry et al., which is incorporated by referencein its entirety herein, teaches a non-azide formulation with mica. Theaddition of mica in the range of 5-25% by weight to a non azide basedgas generant reduces the amount of solid particles exiting the inflatorand also reduces the amount of production of undesired toxic gases suchas nitrogen oxides (NO_(x)) and carbon monoxide (CO). The preferred gasgenerant in U.S. Pat. No. 6,071,364 is 5-amino tetrazole for the fuel,potassium nitrate and strontium nitrate for the oxidizer, and mica. Thisgas generant has a good burn rate velocity and ballistic properties,however this formulation also has some drawbacks, namely itshygroscopicity.

[0005] The present invention relates to an improved gas generantformulation containing mica that is less hygroscopic.

SUMMARY OF THE INVENTION

[0006] The gas generant composition of the present invention comprisesbetween about 15 and 70 wt % nitroguanidine, between about 20 and 80 wt.% oxidizer, and between about 5 and about 25 wt. % of mica. Thepreferred composition comprises about 50 wt. % of nitroguanidine.

DETAILED DESCRIPTION OF THE INVENTION

[0007] The present invention is an improvement of the gas generantcomposition taught in U.S. Pat. No. 6,071,364. The preferred compositionin U.S. Pat. No. 6,071,364 includes 5-amino tetrazole, potassiumnitrate, strontium nitrate, and mica. 5-amino tetrazole is hygroscopicand thus the gas generant containing 5-amino tetrazole absorbs moisture.One molecule of 5-amino tetrazole crystallizes with one molecule ofwater to from a monohydrate structure. The problem associated with thehydration of 5-amino tetrazole is that it has a crystal structure changeassociated with the addition of water. The anhydrous 5-amino tetrazolehas one crystal habit while the monohydrate has another. This problem iscompounded by the fact that water is easily removed form the hydrated5-amino tetrazole at relatively low temperatures. The movement of waterinto and out of the crystal and the associated habit change results in adisintegration of the propellant tablet's integrity. A loss of tabletintegrity may result in loss of density and crumbling, which can cause aloss of ballistic control.

[0008] The gas generants of the present invention containnitroguanidine, an oxidizer, mica, a processing aid, and a burn ratecatalyst. Nitroguanidine (CH₄N₄O₂) is a highly energetic fuel rich innitrogen; nitroguanidine has a low negative oxygen balance (−30.7%). Thegas generant composition in accordance with the present inventioncomprises between about 15 wt. % and 70 wt. % nitroguanidine with thepreferred composition containing about 50 wt. %.

[0009] Nitroguanidine is less hygroscopic than 5-amino-tetrazole, andthus the gas generant according to the present invention absorbs lessmoisture. Another advantage of the formulation in the present inventionis the production of less noxious gases than the gas generant with5-amino tetrazole. A possible explanation for the difference is thepresence of two oxygen atoms on each nitroguanidine molecule allowing ahigher collision frequency probability between the oxygen (serving asthe oxidizer) and the carbon during the combustion process. This is madepossible by the fact that the oxygen of the nitroguanidine is attachedto a nitrogen two atoms away from the carbon. On the other hand, 5-aminotetrazole does not contain any oxygen atoms. Another benefit of thepresent invention is the improved gas conversion efficiency. Similarly,this property can be explained by the presence of the oxygen atoms onthe nitroguanidine molecule. Since a portion of the fuel serves as anoxidizer, less potassium nitrate/strontium nitrate mixture needs to beadded to the fuel. The metal ions from the oxidizers are responsible forlowering the gas conversion rate because they form solid oxideparticles, and thus the formulation in the present invention, which hasless oxidizer than a 5 amino-tetrazole based gas generant will produce ahigher percentage of combustion gas.

[0010] Unprocessed nitroguanidine has at least two distinct nativecrystal arrangements: alpha and beta. In the alpha arrangement, thecrystals have a long white lustrous needle appearance and are tough.While the beta arrangement has crystals in the shape of thin elongatedplates. The alpha arrangement is the desired crystal arrangement forapplications in the propellant and explosive industries.

[0011] When nitroguanidine (alpha arrangement) is pressed into a pelletor tablet, its needles bend or become distorted. During the standardtest of thermal cycling, the energy supplied to the gas generant causesthe nitroguanidine needles to revert back to their original geometry ornative conformation. This results in the pellets growing in size. Onesolution to the foregoing problem is to add a binder to the gasgenerant. The binder prevents the gas generant pellet from growingduring thermal cycling by securing the nitroguanidine needles to theirreduced geometry. Another means of stabilizing the size or density ofthe gas generant containing nitroguanidine is by grinding thenitroguanidine to amorphous crumbs. A suitable grinding machine for thisoperation is the Palla mill or the vibrating ball mill. The process ofgrinding nitroguanidine is discussed in co-owned published patentapplication 2002 0096236 A1, which is incorporated herein in itsentirety by reference.

[0012] Oxidizers useful in the composition of the present inventioninclude the alkaline earth metal nitrates, alkaline metal nitrates,chlorates, perchlorates, and oxides. The preferred oxidizer system inthe present invention is a mixture of potassium nitrate and strontiumnitrate.

[0013] Another component of the composition in the present invention isa processing aid. Those skilled in the art understand that depending onthe particular oxidizers and fuels utilized, certain processing aids maybe helpful in removing the gas generants from the pellet punch duringpelletizing of the gas generant. Examples of processing aids are silicaand boron nitride, but other processing aids may be employed.

[0014] Mica is another component added to the gas generant. Mica isdiscussed in a co-owned patent, U.S. Pat. No. 6,071,364, which isincorporated herein in its entirety. Mica is a group of minerals in thephyllosilicate subclass, and since micas are true phyllosilicates, theyare composed of sheets of silicate tetrahedrons. The minerals in themica group are characterized are constructed of extremely thin cleavageflakes and characterized by near perfect basal cleavage, and a highdegree of flexibility, elasticity, and toughness. The various micas,although structurally similar, vary in chemical composition. Theproperties of mica derive from the periodicity of weak chemical bondingalternating with strong bonding. Representative of the minerals of themica group are muscovite, phlogopite, biotite, lepidolite, and othersuch as fluorophlogopite. In general, the silicon to aluminum ratio isabout 3:1. Any naturally occurring mica is useful in the gas generantcomposition of the present invention. However, those micas containinghalogen atoms such as lepidolite and fluorophlogopite are not preferred.The presence of halogen atoms in certain of the mica group minerals mayresult in the production of combustion gases containing undesirablehalogen ions. The mica useful in the present invention is ground havinga particle size ranging from 2 to 100 microns. This ground mica is alsoreferred to as flake mica. In the present invention muscovite mica witha particle size in the range of 2-25 microns is preferred.

[0015] Mica is employed in the present invention to reduce the amount ofsolid particles exiting the airbag inflator. The preferred oxidizersystem in the present invention comprises a mixture of potassium nitrateand strontium nitrate. During the combustion of the gas generant, themetal ions from the oxidizer system forms various metal oxidizes. Micais employed to reduce the amount of metal oxides from exiting theinflator. It is theorized that mica reacts with the metal oxides andmetal ions to yield a product that condenses on the metal filter. Thus,the metal filter for an inflator acts as a heat sink to limit the amountof solid particles that exit the inflator.

[0016] A burn rate catalyst or enhancer is optionally added to thecomposition of the present invention to increase the combustion rate.Some examples of burn rate catalysts include metallic aluminum, copperII oxide, and metallic silicon. The burn rate for a gas generantcontaining nitroguanidine is a little less than the burn rate for a gasgenerant containing 5-amino-tetrazole. In order for the nitroguanidinebased gas generant to have similar ballistic properties to5-amino-tetrazole, the combustion pressure of the inflator needs to beabout 6897 kPa (1000 psi) greater. The preferred burn rate catalyst inthe present invention is copper II oxide.

EXAMPLE 1 Preparation of Gas Generant

[0017] TABLE 1 provides the compositions for three samples. Sample 1 isdirected to a 5 amino tetrazole (5-ATZ) formulation discussed inco-owned patent U.S. Pat. No. 6,071,364, and the process of mixing thegas generant is discussed thereto and is incorporated herein byreference.

[0018] TABLE 1 also provides samples 2 and 3 which are directed to a gasgenerant having nitroguanidine (NQ) as the fuel. The difference betweensamples 2 and 3 is the choice of burn rate catalyst. The process ofmixing sample 2 is virtually identical to mixing sample 3 except for thetype of burn rate catalyst added.

[0019] Samples 2 and 3 were prepared by individually grinding all of thecomponents, except for the mica and respective burn rate catalysts. Thenitroguanidine was ground to amorphous crumbs through a Palla-mill. Thestrontium and potassium nitrates were ground in a fluid energy mill. Themica used was Micro Mica 3000 (muscovite) obtained from the Charles B.Chrystal Co., Inc. of New York, N.Y., U.S.A. It was a finely dividedmica having a bulk density of about 12.4 lbs./cubic foot and a specificgravity of about 2.8.

[0020] The ground NQ and ground strontium and potassium nitrates weremixed with the mica and respective burn rate catalyst to homogenize theformulation. The mixture was then placed in a plough-type mixer andabout 15% by wt. water was add to form an agglomerated material that wasthen passed through a granulator with an 8 mesh screen.

[0021] The granules were placed on a tray and dried at 120° C. in anexplosion proof oven for about 3 hours. The water content after dryingwas between 0.5 and 1% by wt. The dried granules were then passedthrough the granulator using a 20 mesh screen. The gas generant was thenpelletized with a rotary TABLE 1 Sample # 5-ATZ NQ KNO₃ Sr(NO₃)₂ Mica BNSi CuO 1 32 8 44 16 1 2 50 6 35 8 1 1 3 50 6 35 7 1 2

EXAMPLE 2 Tests on the Gas Generant

[0022] For the burn rate tests, the samples were prepared in a similarfashion to the procedure set forth in Example 1 except the componentswere ground separately, dry blended, and pressed into strands fortesting. The strands had a rectangular shape with about 10.16 cm inlength and about 0.63 cm on each side. The sides of each strand werecoated with an epoxy-based adhesive. Strands were placed in a strandburner bomb. The bomb was equipped with a pressure transducer, acousticdevices, and mechanical wire burn through recorders. The strands wereignited at 7585 kPa (1100 psi), and pressure versus time was recorded.The acoustic and mechanical devices calculated burning time. Burningrate was determined by dividing the length of each strand by its burningtime. The average burn rate for six strands for each sample is presentedin TABLE 2. The burn rates for samples 2 and 3 are a little less thanthe burn rate for sample 1.

[0023] The moisture absorption test was utilized to determine the amountof moisture the samples absorbed. The samples were pre-weighed and thenexposed to the following conditions: 50% relative humidity and 22.2° C.The samples were weighed again after 6 hours to determine moisture gain.The results from this experiment are displayed in TABLE 2. Samples 2 and3 each had less than half the moisture sensitivity of sample 1.

[0024] To arrive at the conversion efficiency, the thermodynamic program“NEWPEP” is employed. “NEWPEP” is based on the PEP program described ina Naval Weapons Center Report entitled, “Theoretical Computations ofEquilibrium Composition, Thermodynamic Composition, ThermodynamicProperties, and Performance Characteristics of Propellant Systems,”published in 1960, 1979, and 1990. This program is in the public domainand is readily available to those in the industry.

[0025] After entering into said program the weight of the ingredients inthe gas generant and the possible species that can be formed by theburning of the gas generant, the program can calculate the number ofmoles for each theoretical combustion product. Conversion efficiency iscalculated by dividing the total number of moles of the gaseous reactionproducts by the total number of moles of reaction products, and thenthis quotient is multiplied by 100 to give the conversion efficiencyvalue as a percentage. The percentage equivalence to the conversionefficiencies for the three samples is presented in TABLE 2. Samples 2and 3 had a larger conversion efficiency than sample 1. TABLE 2 MoistureConversion Sample Burn rate sensitivity efficiency 1 2.84 cm./sec. 0.30%64% (1.12 in/sec) 2 2.09 cm./sec. 0.12% 71% (0.823 in./sec.) 3 2.21cm./sec. 0.12% 68% (0.869 in./sec.)

[0026] Airborne particulates and toxic gas data were obtained for thevarious samples. In order to obtain said data, the gas generants wereplaced in identical single stage inflators, which were added toidentical airbag modules. The tests were conducted in a 100 cubic foottest chamber. This test is designed to simulate the interior volume ofthe standard automobile. The test equipment consisted of a 100 cubicfoot steel chamber containing a steering wheel simulator. To the chamberwas attached a vacuum pump, a bubble flow meter, filters, and a FT/IR(Fourier Transform Infrared Spectroscopy) gas analyzer. The inflator wasattached to the simulated steering wheel assembly within the chamber,the chamber was sealed and the gas generant ignited. Airborneparticulate production was measured by filtering post-ignition air fromthe chamber through a fine filter and measuring the weight gained by thefilter. The CO and NO_(x) levels of the gases produced were analyzed byusing FTIR at intervals of before deployment (background), 1, 5, 10, 15,and 20 minutes after deployment. Samples were transferred directly tothe FTIR gas cell from the 100 cubic foot chamber via six feet of ¼ inchOD fluoropolymer tubing.

[0027] The amount of toxic gas and total airborne particulates forsample 2 was significantly lower than sample 1. TABLE 3 Total airborneSample CO (ppm) NO_(x) (ppm) particulates (mg/m³) 1 172 15 53 2 84 8 323 173 7 50

We claim:
 1. A gas generant comprising: (a) 15-70 wt. % ofnitroguanidine; (b) 20-80 wt. % of an oxidizer selected from transitionmetal oxides; alkali metal nitrates, chlorates and perchlorates;alkaline earth metal nitrates, chlorates and perchlorates; and mixturesthereof; and (c) greater than 5 and less than 25 wt. % mica.
 2. The gasgenerant of claim 1 further comprising greater than 0 to about 3 wt. %of a burn rate catalyst wherein the burn rate catalyst is a chemicalselected from the group consisting of copper 11 oxide, metal silicon, ormetal aluminum.
 3. The gas generant of claim 2 wherein the preferredburn rate catalyst is copper II oxide and the preferred amount is 1 wt.%.
 4. The gas generant of claim 1 wherein the preferred wt. % fornitroguanidine is
 50. 5. The gas generant of claim 1 wherein thepreferred oxidizer is a mixture of potassium nitrate and strontiumnitrate.
 6. The gas generant of claim 5 wherein the preferred wt. % forpotassium nitrate is 6 and the preferred wt. % for strontium nitrate is35.
 7. The gas generant of claim 1 wherein the preferred wt. % for micais 8.